Skeletal Muscle Lab Questions

For extra help on the graph, see this post:

http://bio131lfall2011.blogspot.com/2011/10/skeletal-muscle-graph.html

Use your data from page 60 of your lab manual for this graph.

Lab 3 question 1

Help for Question 1:

Please remember, reflexes and reactions are two completely different things. A reflex is already programmed into your body and there is nothing you can do to remove it. A reaction is learned and thus can be un-learned. A learned reaction will never become a true reflex. The case with Pavlov's dog is a learned reaction, not a reflex; though some people call it a reflex, it is not a reflex in the scientific sense. 

As for the neuronal sequence involved, refer to page 42 of your lab manual. The answer to question 1 should have at least this much detail.

Question 1 B

Q) I just had a question on part "B" on number 1. In our notes it says that a larger stimulus will create a higher frequency of action potentials, which is what I was going to base my answer off of, but what is considered a large stimulus? How do you know if something is a large stimulus?


A) The strength of the stimulus is not important for this question. 

You just need to know if it will be easier or harder for this cell to reach its threshold voltage to generate an action potential. 

Does the change in K+ concentration in the ECF move the cells potential towards or away from its threshold voltage?

Action Potentials

This website may help you understand action potentials better:

http://outreach.mcb.harvard.edu/animations/actionpotential.swf

Homework 1

Q) for question 2, saying the cell membrane hyperpolarizes means that the cell is becoming more negative right? because repolarizing means that the cell is becoming more negative, BUT had to depolarize first.   

A) Hyperpolarizes means that the difference between the ECF and ICF is becoming larger than at rest. So if the cell is already negative, then when it hyperpolarizes it becomes more negative.

After a cell depolarizes (the difference between ICF and ECF became smaller, went towards 0) the cell must "repolarize" meaning that it must build up its normal charge difference between the ICF and ECF (this is done with the Na/K pump). 

This is the way I think of this concept:

Lets pretend that the ECF and the ICF are the two poles on a magnet. As you know, a magnet is polarized and has a positive end and a negative end. There is a difference in the charges of the two poles.

If the magnet was hyperpolarized, that would mean that the two poles would have much stronger charges than normal (hyper= increase). The positive pole would become more positive and the negative pole more negative. Thus the charge difference between the two poles would become much larger than the original magnet. This magnet would become super strong.

If the same magnet was depolarized, that would mean that the two poles would have much weaker charges than normal (de=less). The positive pole would become less positive and the negative pole less negative. Thus the charge difference between the two poles would become much less than the original magnet and the magnet's strength would be very weak. The charge difference between the two poles of the magnet could even become 0 and the magnet would not be a magnet anymore, just a neutrally charged blob of metal.

If you were to repolarize that weak, depolarized magnet, that would mean that you are adding charge to the poles, making the charges at the poles stronger. Repolarize means that you will restore the charges of the two poles (and the difference in charge between the two poles) to the way it was originally, thus bringing the magnet back to its original strength. 

So that's how I remember what the words polarize, depolarize, hyperpolarize, and repolarize mean.  When thinking about cells specifically, think more about the difference of the charges between the ICF and the ECF, and not so much about which one is negative and which is positive (since the ECF is always set to 0).

Homework 1, question 2

Q) I am working on the membrane transport homework and I have a question regarding number 2.

Do I need to know the concentration of Cl in order to solve the problem because it is not given?

A) Yes, you'll need to look up the normal concentration of Cl- inside and outside of normal cells. You may be able to find it in a text book, and definitely somewhere on the internet. You can check with me to make sure it is correct.

Osmolarity and Tonicity Concepts

Q)  I'm trying to wrap my head around osmolarity. when comparing penetrating solutes to non penetrating solutes, since the penetrating solutes are allowed to move through the membrane - wouldn't that be diffusion, since the solutes are freely moving from high to low concentration - thus water would not need to move in or out of the cell? I understand that with non penetrating solutes, since the solutes are not able to move freely, water moves to dilute the more concentrated non penetrating solutes. What i am confused about is why the movement of penetrating solutes is tied with osmolarity...to clarify my viewpoint - the movement of penetrating particles from high to low is "diffusion"...


I'm reading the book as i'm emailing you, but this might answer my question...

if you have a mixture of penetrating solutes and non penetrating solutes then thats where the penetrating solutes come into play with osmolarity - which in this case tells us about tonicity? whereas if you just have penetrating solutes in a cell and solution, the movement of the penetrating solutes is just simple diffusion and thus tonicity is irrelevant...?

sorry if this is confusing, but i'm having a hard time trying to tie everything together

A)  Yes, penetrating particles move from one compartment to the other by simple, passive diffusion. 

Osmolarity depends on the number of particles in each compartment, not on whether they are penetrating or non-penetrating.

Tonicity (whether or not the cell shrinks or swells) is dependent on the ICF and ECF concentration of non-penetrating particles. If there areonly penetrating particles present, then there will be no effect on tonicity.

However, if there is a mixture of penetrating and non-penetrating particles, both will may have an effect on tonicity.

If you remember the two examples in class today, I had a cell in an iso-osmotic but hypotonic solution, and another in a hypo-osmotic and isotonic solution. In both examples there were penetrating and non-penetrating particles. In those examples we had to see what was going on with the penetrating particles to understand the effect of tonicity on the cell. 

Also, remember that osmolarity of a solution (whether you call a solution iso-, hypo-, or hyperosmotic) depends on the number of particles BEFORE putting the cell into solution, whereas tonicity of a solution (whether you call it iso-, hypo-, or hypertonic) depends on what happens to the cell AFTER it is put in solution and the particles balance themselves out between the two compartments.